Duramax Diesels Forum Truck of the Week
  #1  
Old 06-11-2019, 04:16 AM
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Default What gear do you guys usually trap a 1/4 in?

Finally got my truck to the track for the first full run on it (broke the crank last year) I always put it in manual and select 5th to keep it out of 6th.. I almost swear it was on the rev limiter going through the traps granted this truck is faster then my last one but I always raced it keeping it in 5th and it had 28 inch tall sticks on it and I never had a problem but only trapping 115... my current truck judging by looking at a lot of the high ten second time slips on here my 1/8 mph was 98.99 which is like one off from a lot of the ten second passes but only trapped a 11.04 at 121.. tires on this truck are 315/45/16 drag radials. I figure shifting to 6th that late in the run would slow it down.... what do you guys think? Thanks Brendon
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Old 06-11-2019, 04:26 AM
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Hold onto 5th if possible. No sense in defueling for a 6th gear shift. Figure out what RPM you need to hold 5th for your desired trap speed based on tire size and ratios, make sure it is reasonable for your valve train.

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Old 06-11-2019, 06:43 AM
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It really is case-by-case. If your HP starts to fall off too far as you push RPM, it might be worth the defuel and shift to get back in the powerband. You need to look at the average HP output over the last part of the run. That means how badly does your engine choke out as you rev, and how much defuel do you have to deal with? Each of my built transmissions has had a different amount of defuel at WOT.

At WOT, my current trans will only defuel the 1-2 shift, and every other shift holds 120mm³, so I let it shift 4-5 at 95 MPH then cross the line a second later at 104 MPH. It can do 104 MPH in 4th, but I am falling off the HP curve and it is actually slower. But this math all changes if there is a large defuel for the shift.

If you let it shift to 6th and it barely gets into 6th before you cross the line, probably not worth it, just stay in 5th and rev it out. If you are holding 5th for what seems like forever, and it feels like power is dropping off, you might want to let it hit 6th. You can look at logs and see how much defuel you have, look at a dyno pull and see how badly your HP curve falls off as you rev, and do the math. Or, you can just do 2 runs and see if hitting 6th makes you faster or slower
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Old 06-11-2019, 06:59 AM
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I have a 5 speed truck. In the 1/4 it took 5th but I don't know at what point. I ran a 7.498 @ 100.90mph in the 1/8th mile and ran a 11.391 @ 126.85mph in the 1/4 mile.




I know it needed 5th but my setup doesn't pull hard off the line due to the big single, big stall converter, and 35" old BFG mud terrains.
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Old 06-11-2019, 08:42 AM
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Sounds like regearing the rear end may help you a lot
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Old 06-11-2019, 09:06 AM
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Quote:
Originally Posted by 2004LB7 View Post
Sounds like regearing the rear end may help you a lot
I hope his truck is 2wd
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Old 06-11-2019, 09:11 AM
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I did only get to make one run and I forgot to data log it...and for how small my turbos are I’m sure it was out of power I just didn’t expect it to run out of rpm or gear whichever .. might try to get to the track again tomorrow night and see what 6th gear does
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Old 06-11-2019, 09:12 AM
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No it’s 4wd that’s why re gearing probably won’t happen
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17 Silverado L5P ltz custom sport stock
06 gmc arp studs and mains Callie’s crank corrillos mahle Pistons wagler alt fire street cam.big dipper trans precision converter. dual fuelers 60 overs 75 over 66, 5600lbs... track times coming soon
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Old 06-11-2019, 09:29 AM
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I would keep it out of 6th. In 6th you're putting a bigger load on the engine with the rear planet spinning.
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Old 06-11-2019, 09:38 AM
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A 315/45R16 drag radial is only 27.16" tall, that may also be part of your problem.


What is your limiter set at? Do you have a dyno graph to see where power carries to or how far past peak it nosedives?
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Current Rides:
1) Toy: 2005 LLY ECSB, Leather, Sunroof
-1340hp/2200ft-lbs @ ATS Diesel (06/04/19)
-Tuned by Zach @ Starlite
-Built Motor, Forced Inductions Billet ETR Gen 4 S480/96/1.10 T6 w/V2 Race Cover, Modded Exergy 250% Injectors, Exergy 14mm Street Pump/LBZ CP3, Goerend Q Converter, Billet Trans, Fuelab Supply Pump/Reg, RCD & Beans Sumps, Front E-Locker, TrueTrac, Yukon 4340 Rear Shafts, Traction Bars, DHD Center Link Brace, DHD Lower Crossmember Support


2) DD/Tow Rig: 2002 LB7 CCSB, Leather, Loaded
-Tuned by Starlite
-Bosch NEW Injectors, FASS 150, 3" Downpipe, 4" Downpipe Back, RCD Sump, PPE Sleeves, DHD Centerlink Brace, DHD Lower Crossmember Support, Steering Wheel Radio Control Swap
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Old 06-11-2019, 10:03 AM
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When you hit 6th it will feel like you lost all power. Your tire height is a issue but you should still see higher trap speeds.
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Old 06-11-2019, 11:38 AM
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I have never had it on a dyno and I’m not sure where mark set the limiter... I’m honestly starting to second guess myself to it being in 4th gear instead of 5th. After I did my burnout I was creeping to the staging light and a guy on the track backed me up to wait for the guy beside me... so I had to pull it back in to M and then select the gear again... I feel like I should get more then 121 out of 5th gear.... guess I might have to hit a back road tonight and find out.
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06 Silverado lbz 11.77 @115 sold
17 Silverado L5P ltz custom sport stock
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Old 06-11-2019, 01:52 PM
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Well just took it for a ride it was definitely in 5th gear
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Old 06-11-2019, 04:16 PM
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And I also measured my tires on the truck and they are only about 26 and a half inches tall... looks like I’m gonna need some new tires.
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17 Silverado L5P ltz custom sport stock
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Old 06-11-2019, 05:25 PM
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Quote:
Originally Posted by DAVe3283 View Post
It really is case-by-case. If your HP starts to fall off too far as you push RPM, it might be worth the defuel and shift to get back in the powerband. You need to look at the average HP output over the last part of the run. That means how badly does your engine choke out as you rev, and how much defuel do you have to deal with? Each of my built transmissions has had a different amount of defuel at WOT.

At WOT, my current trans will only defuel the 1-2 shift, and every other shift holds 120mm³, so I let it shift 4-5 at 95 MPH then cross the line a second later at 104 MPH. It can do 104 MPH in 4th, but I am falling off the HP curve and it is actually slower. But this math all changes if there is a large defuel for the shift.

If you let it shift to 6th and it barely gets into 6th before you cross the line, probably not worth it, just stay in 5th and rev it out. If you are holding 5th for what seems like forever, and it feels like power is dropping off, you might want to let it hit 6th. You can look at logs and see how much defuel you have, look at a dyno pull and see how badly your HP curve falls off as you rev, and do the math. Or, you can just do 2 runs and see if hitting 6th makes you faster or slower

35" tire? Mines at the top of 4th at the 1/8mi ~85mph
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